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The •OH scavenging effect of bromide ions on the yield of H[subscript 2]O[subscript 2] in the radiolysis of water by [superscript 60]Co γ-rays and tritium β-particles at room temperature : a Monte Carlo simulation study

Abstract

Abstract: Monte Carlo simulations were used here to compare the radiation chemistry of pure water and aqueous bromide solutions after irradiation with two different types of radiation, namely, tritium β-electrons (~7.8 keV) and [superscript 60]Co γ-rays/fast electron (~1 MeV) or high energy protons. Bromide ions (Br-) are known to be selective scavengers of hydroxyl radicals •OH precursors of hydrogen peroxide H[subscript 2]O[subscript 2]. These simulations thus allowed us to determine the yields (or G-values) of H[subscript 2]O[subscript 2] in the radiolysis of dilute aqueous bromide solutions by the two types of radiations studied, the first with low linear energy transfer (LET) (~0.3 keV/μm) and the second with high LET (~6 keV/μm) at 25 °C. This study was carried out under a wide range of Br- concentrations both in the presence and the absence of oxygen. Simulations clearly showed that irradiation by tritium β-electrons favored a clear increase in G(H[subscript 2]O[subscript 2]) compared to [superscript 60]Co γ-rays. We found that these changes could be related to differences in the initial spatial distributions of radiolytic species (i.e., the structure of the electron tracks, the low-energy β-electrons of tritium depositing their energy as cylindrical “short tracks” and the energetic Compton electrons produced by γ-radiolysis forming mainly spherical “spurs”). Moreover, simulations also showed that the presence of oxygen, a very good scavenger of hydrated electrons (e-[subscript aq]) and H• atoms on the 10[superscript-7] s time scale (i.e., before the end of spur expansion), protected H[subscript 2]O[subscript 2] from further reactions with these species in the homogeneous stage of radiolysis. This protection against e-[subscript aq] and H• atoms therefore led to an increase in the H[subscript 2]O[subscript 2] yields at long times, as seen experimentally. Finally, for both deaerated and aerated solutions, the H[subscript 2]O[subscript 2] yield in tritium β-radiolysis was found to be more easily suppressed than in the case of cobalt-60 γ-radiolysis, and interpreted by the quantitatively different chemistry between short tracks and spurs. These differences in the scavengeability of H[subscript 2]O[subscript 2] precursors in passing from low-LET [superscript 60]Co γ-ray to high-LET tritium β-electron irradiation were in good agreement with experimental data, thereby lending strong support to the picture of tritium-β radiolysis in terms of short tracks of high local LET.